1. Field of the Invention
The present invention relates generally to dewatering mixtures of fine-grained material and water, and more specifically to dredged sediment and sludge dewatering by vacuum bag method, which decreases time and complexity of water removal from dredged sediment and sludge.
2. Discussion of the Prior Art
Dredging spoils were historically disposed wherever most conveniently dumped, on land or at sea. Increasing regulation in recent decades has required controlled disposal of contaminated sediment on land. This has led to development of economical dewatering methods for all of the sediment material types, except those containing significant amounts of fine-grained organic matter. Sediment from marine, brackish, and fresh-water bodies may consist of a wide range of material types, including sand (with or without gravel), inorganic silt, clay, organic silt, and peat (fibrous as well as amorphous).
Dredged sand and inorganic silt have relatively low water content after a short amount of settling time. Water readily drains from sand by gravity alone. The small inter-granular pore spaces within inorganic silt results in capillary tension that resists dewatering, but that can be overcome by centrifuging or vacuum belt operations to remove the relatively small amount of water that remains after settling, if that is required. Wet clay is commonly stabilized by the addition of lime, which chemically reacts with both the interstitial water and the alumino-silicates that form the clay minerals. Fibrous peat and some waste sludge materials can de-watered effectively by belt press or mechanical press. Air-drying can be effective with any dredged sediment or sludge material, but may be countered by rainfall unless the large area required for layout and disking of the material is covered. Dewatering by heating is not often economical due to large energy cost required to elevate the temperature of and evaporate the high water content of organic sediment and sludge.
In recent years, large amounts of organic silt have been dredged and will continue to be removed from many harbors, rivers and estuaries because of contamination with heavy metals (Cadmium, Lead, and Mercury) and polychlorinated biphenyls (PCB's) from past, uncontrolled industrial discharges. The fine-grained material comprising organic silt accumulates heavy metals, and the organic matter adsorbs oily PCB's. The dredged material may be rendered environmentally safe using high temperature incineration, but at present it is more commonly disposed in landfills without treatment. Both fates require removal of much of the very high water content that is characteristic of organic silt, particularly after the underwater disturbance inherent with both mechanical and hydraulic dredging. Even after months of gravity settling, the water content of dredged organic silt is commonly in the range of 100% to 200% on a weight basis and the material is of semi-liquid consistency. Amorphous (non-fibrous) peat may also be present at even higher water content. In geotechnical engineering terms, the “liquidity index” of dredged organic silt after gravity dewatering often remains in the value range of one to two.
Similar to dredged organic sediment, paper mill and sewage treatment plant sludge have high organic and water content and present similar challenges for handling and disposal. Where it is not practical to beneficially apply liquid sewage sludge to farm fields, the sludge must be dewatered before disposal. In many cases this is accomplished with mechanical or belt presses. Otherwise, liquid to semi-liquid dredged sediment and sludge that have high water content are difficult to handle, expensive to incinerate, and are too unstable physically to landfill.
Considerable effort and expense is incurred with the use of various additives to adsorb or react with the excess water in dredged organic sediment. This actually adds to the weight and volume that must be disposed, which is a strong disadvantage because of the very large quantities involved. Several hundred thousand cubic yards of sediment is commonly removed in the course of each of the numerous major environmental remediation projects underway across the nation at this time.
Although extensive research and field effort has been made in recent years by dredging companies, geotechnical engineers, and research institutions, environmental cleanup projects are falling behind the committed schedules and expenses are exceeding budgets by large amounts because effective dewatering methods have not been developed for sediment with substantial organic content. But the compressibility, high water content, and moderately low permeability of organic sediment and sludge makes dewatering by consolidation feasible, provided a practical method of applying the required load to a semi-liquid material is used.
The potential effectiveness of rigid container mechanical pressing is obvious. This is done commercially on limited quantities to dewater various sludge wastes. However, moderately low permeability organic silt requires impracticably long times for mechanical pressing of the large volumes of dredged sediment. Mechanical pressing is analogous to the small scale “consolidation” test commonly performed in geotechnical labs. This test uses rigid confinement on all sides of the test specimen but the top, where external mechanical load (with weights and lever arms) presses on a rigid, but porous “stone” to compress the material.
Equations, well known to geotechnical engineers, describing the amount of soil matrix compression that occurs under net pressure (called consolidation), and the time it takes for that compression to occur, are the same irrespective of how the stress is applied. In addition to the obviousness of mechanical pressing, soil scientists and geotechnical engineers are aware that applying vacuum directly to interstitial (pore) water exerts compressive stress on the soil matrix. This is referred to as “pore water tension”, the potential for which is controlled by the effective pore diameter of the soil matrix. Plant roots exert this effect on soil and air drying has the same effect, to the extreme in clay.
Pore water tension is used to stabilize excavations in inorganic silt below the water table. This is accomplished by applying vacuum to “sand points” driven into water laden silt strata. In similar configuration, vacuum is applied to wells installed in soil to enhance removal of liquid contaminants and contaminated ground water, as well as move vapor through the soil to promote natural biodegradation of organic contaminants.
In contrast, soil scientists and geotechnical engineers are not commonly familiar with application of vacuum to the interior of an air impermeable but flexible membrane enclosure, which causes atmospheric pressure acting on the outside of the membrane to exert uniform compression on any material contained within. However, this technique has been used for decades in the composite materials manufacturing industry. It can be adapted to provide a practical dewatering method for dredged organic sediment and organic sludge. In manufacturing, it is called “vacuum bagging” and is used to produce high quality laminates of fiberglass, carbon, or other reinforcing fibers bound with cementing resins. Atmospheric pressure is used to apply uniform pressure to the layers of fiber and resin to increase density, resulting in high strength after curing. This is accomplished by applying vacuum to absorbent and porous material that overlays or surrounds the piece being made, all of which is sealed from the atmosphere by placing it in a plastic bag or air impermeable membrane of appropriate size. Flat sheets of composite material are often made by laying the absorbent/porous material and impermeable membrane over the layers of fiber and liquid resin placed on a table. A bead of caulk on the table surrounding the uncured composite sandwich seals the interior (to which vacuum is applied) from the atmosphere. A variation using a porous table with vacuum applied underneath is also common.
In addition to the commonly practiced art of vacuum bagging in composite materials manufacturing, the other relevant prior art is vacuum assisted, in-situ consolidation of soil. That potential was initially described by a German engineer in 1930 and was initially used in 1952. Although that practice has not since become common, vacuum assisted consolidation has been used from time to time in various countries to “pre-consolidate” building sites on soft ground in order to avoid excessive, gradual subsidence. In this in-situ application, vacuum is applied to vertical drains installed in the soil as well as the over the ground surface by covering the area with an air impermeable membrane. Perimeter “curtain walls” of slurry are often required to provide lateral isolation of the consolidation zone. The vacuum increases the in-situ soil stress and induces consolidation, as disclosed in Menard Vacuum Consolidation, ISSMFE-TC-17. All of these applications required use of vents or drain elements installed into the ground, and differ from the use described in this patent application from prior process art in at least that respect. Most of the applications also applied gravity weight (piled soil) surcharge in addition to vacuum-induced atmospheric pressure to accomplish the required amount of pre-consolidation of soft ground building sites.
The potential for vacuum assisted consolidation of dredged sediment or waste sludge placed in a landfill is well described in technical publications by Professor Thevanayagam of the Civil Engineering Department at SUNY, Buffalo, N.Y. In-situ consolidation of hydraulic fill placed under the water is also described. The most comprehensive summary of these applications is his article titled Vacuum-Assisted Consolidation of Coastal and Offshore Dredge Fills (published in ASCE Geotechnical Special Publication No 65). It reviews the fundamental soil mechanics principles and some practicalities applicable to vacuum assisted, in-situ consolidation. All of the applications presented included description of combinations of horizontal and vertical drainage elements installed within the mass of the sediment being dewatered. The use of vacuum for ex-situ, pre-disposal dewatering of sediment, and sludge without use of drainage features or elements embedded within the sediment has not been obvious to practitioners, although there are large numbers of scientists and engineers active in this field.
In recent years, it has become common practice to pump hydraulically dredged sediment as well as sewage sludge into Geotubes® to accomplish some initial separation of water from the fine-grained solids. Geotubes® are simply very large, sausage-shaped skins made of water pervious “geotextile” material. Within several months after filling, the water content of the contained organic sediment is stabilized at about 100% to 200% on a weight basis. Water content can be much higher in sewage sludge or if amorphous peat is present in dredged sediment. In any case, organic materials are typically in semi-liquid condition after gravity dewatering in Geotubes®. Geotube® is a registered trademark of Nicolon Corporation of Pendergrass, Ga.
After initial gravity de-watering, fly ash and/or quick lime is sometimes added to absorb and react with the remaining excess water to provide soil-like consistency prior to land filling dredged sediment. However, this retains all the water weight, to which the additives add about 25% to the already large weight of the sediment with its excess water. The weight increase and cost of additives results in disposal costs that are often twice what would be incurred if vacuum bag dewatering were used to remove water, decrease the disposal weight and volume, and provide workable consistency. Without dewatering or stabilization treatment, disposal of semi-liquid dredgings at landfills can be triple the standard disposal fee of $20/ton, amounting to about $60/ton.
Sewage and paper mill sludge contained in Geotubes® can be further de-watered by air drying if covered from precipitation, but that process may take too long.
Accordingly, there is a clearly felt need in the art for dredged sediment and sludge dewatering by vacuum bag method, which decreases the amount of time and cost required to dewater materials, particularly those with substantial fine-grained organic content, and which does not require internal drains placed within the sediment or waste sludge mass.